Four hundred years ago, the Paradesi synagogue was founded here, ordered by royal decree to abut an ancient Hindu temple. Although the arrangement may seem startling to Westerners, to Indians it is a way of life.

Since the Jews' arrival in India more than 2,000 years, the sounds of mantras have commingled with Hebrew chants, especially here in Cochin, a coastal city in the state of Kerala.

The Jews here are a highly educated, secure, and prosperous group. Yet they are leaving. Emigration to Israel has decimated this proud community. The Jewish population here once numbered in the thousands; today it is reduced to 33 people. Observers predict that in five years there will be no Jews left in Cochin.

With no experience of persecution - save for a brief period during Portuguese rule - Jews found in India a haven in which their traditions flourished and developed unique forms. But many have opted for a harsher life in Israel.

The origins of this community are as ancient as they are obscure. Jews migrated to Kerala State between 2,000 and 2,500 years ago. Official records say ''about 10,000 Jews and Jewesses came to (the) Malabar (coast) and settled ... in Cranganore,'' then a port 20 miles north of Cochin. They quickly took their place alongside other residents, prospering in a variety of sea-related trades.

Quite remarkably, they also were granted an independent principality by a Hindu king, according to an ancient decree inscribed on copper plates that are now kept in the Cochin synagogue. The king appointed a Jewish leader as sovereign prince, and his rule was handed down to his descendants, who governed for more than 1,000 years.

In 1341 a massive flood silted up Cranganore's port. Most of the city's Jews journeyed south to Cochin, where the same flood had created an enormous natural harbor.

When the British seized control in 1798, social and economic pressures led to a gradual weakening of the Cochin Jewish community. Lured by prospects in such rapidly-industrializing cities as Bombay and Calcutta, thousands moved north, where they assimilated into other Jewish settlements. So great was their migration that by the end of British rule in 1947, only about 300 remained.

Today, Cochin's Jew Town, as it is called by everyone, remains a throwback to medieval times. The pungent aromas of cardamom, cloves, and pepper still waft through its narrow, cobblestoned streets. Embellishing the pastel-colored homes that line Synagogue Lane are Stars of David and menorahs (Jewish candelabras), now joined by Muslim crescents, Hindu images, and Christian crosses.

The Paradesi Synagogue contains tiles from China, chandeliers from Belgium, prayer books from Israel, and Torahs (Jewish bibles) copied by local scribes. The ancient copper plates, bequeathing the principality of Cranganore, are withdrawn from the Ark, where the Torahs are stored, as is a ceremonial shofar (ram's horn) of exceptional proportions.

Yet the tranquility of this Old World charm belies deep concern over Jews leaving.

In the center of town stands the regal home of Sattu and Gladys Koder, the undisputed leaders of Cochin Jewry. When Gloria Halegua, Koder's niece, moved to Israel in 1972, she opened the floodgate for a mass exodus. She and her husband, Naphtali, descendant of another eminent family, were expected to assume the elderly Koders' leadership of the community.

''They shouldn't have left,'' said Gladys Koder of Jews leaving Cochin. ''With no persecution here, there was no reason to liquidate the community. I love Israel, too, but it is my spiritualm home.''